Quote from
jh5speed:
Just to note the Dart is a single spool animal - prop is connected to the one (and only) shaft. 2 stage of compressor (centrifugal) and 2 turbine stages though - perhaps that's what you were thinking of?
[Unquote]
Yes... Mea culpa, I've belatedly looked at a cut-away. Shouldn't have relied on memory, 37 years on. You,
ABUKABOY and
barit1 didn't need any help from me, you were spot on.
I think
barit1 is right, a slower advance of the throttles avoids the momentary drop in rpm as the prop comes out of ground-fine into governing mode. On the Herald, the latter was announced by the pitch lever moving forward (provided the controls had been unlocked), and the green ground-fine-pitch lights going out.
For the landing, the Viscount selects its own ground-fine, apparently, by the weight switches. On the Herald, the crew had to remember it. Don't know about the F27.
411A is right that there is a change of note if/when water-meth cuts in. The note gets heavier (more thrust from coarser pitch at the same maximum rpm?) and, on the Herald, heard externally, there was often a strange, slapping sound.
Quote from
dixi188:
Also the -520 series Dart, on Viscounts and F27s, W/M restores power at hight ambient temps so would not be used in the UK very often except on hot summer days, where as the -532 on the Dart Herald boosted power at all temps. so was used more often.
[Unquote]
That would explain why on the Herald (only 2 engines, if my memory hasn't failed again)
we often used it, together with zero flap, to satisfy the Performance-A "WAT" limit (to improve the second-segment climb angle in the engine-out case); even with fairly modest summer temperatures.
[Rather off-topic, "Whispering Giant" was a marketing slogan for the (Proteus) Britannia. Certainly quiet inside, from my (pax) experience. But, as you say, there was a deep rumble in the air, heard externally. The Bristol Proteus was 2-spool (I checked this time!), with axial-flow compressors on the N2, and 2 turbines per spool; but the air flowed forwards while being compressed, then reversing direction into the combustion chambers. This enabled the engine to be shorter (but caused the serious African icing problems early on). When taxying, at idle revs, it made an almost inaudible whisper (observed from the perimeter fence), occasionally interrupted by a "whoosh" - maybe as the props went into superfine pitch when the throttles were completely closed?
So unlike the deafening high-pitched song of the Dart... I would love to have a recording of the Proteus, but doubt finding one on a pop album.]